194 D. J. Osterholt and T. Kelly first successful flight of Roc was achieved, however, there were many challenges in designing, manufacturing, testing, and flying the aircraft. A successful program such as the Roc program requires multiple ground tests to validate and update the finite element models (FEMs) used to predict safe flight, including predictions for flutter analysis, dynamic stability, and loads. With the support of ATA Engineering, Inc., (ATA), the successful ground vibration test (GVT) program for Roc was performed in three phases: (1) a single empennage test focused on the rudders and elevators; (2) a single wing test focused on the ailerons, flap, and engine; and (3) a full-scale aircraft test. The objective of the first and second GVTs was to measure the fundamental modal properties, including the frequency, damping, and mode shape, of each subsystem, and the third, full-scale GVT focused on the global modes of the entire aircraft. The three phases of the test program are discussed, including the challenges associated with each of them. Note that all data provided in this paper is unitless, not real, and presented as typical. 17.2 Phase 1: Empennage Stratolaunch Systems Corporation engineers were initially concerned with verifying the validity of the aft tail section (empennage) of their Roc FEM, so the initial GVT focused only on the right empennage, elevators, and rudders. Figure 17.1 shows the Roc aircraft from the aft. The test focused on the right-side rudder and elevator control surfaces to assess the nonlinearity of control surface modes under different hydraulic configurations. The initial challenge for this phase involved the provision of a shaker stand tall enough and stiff enough to allow proper excitation of the upper and lower rudders. The selection of accelerometer locations was relatively straightforward and resulted in 48 accelerometers arranged on the empennage components. Figure 17.2 shows the upper and lower rudder accelerometer locations, and Fig. 17.3 shows the test display model (TDM) used to visualize the animated mode shapes. The shaker support consisted of a welded steel frame that could be clamped to a large scissor lift as shown in Fig. 17.4. The lift was large and tall enough to allow high-force-level inputs on the upper rudder, lower rudder, and elevators. During the test of the empennage, a new accelerometer cable system was evaluated, which was later implemented in the full-scale aircraft GVT. The custom shaker support worked well and was utilized for the subsequent GVTs. Seventy-seven runs were recorded and ten sets of control surface sweeps performed. The runs consisted of all checkout runs, random excitation, and sine sweeps. A set of control surface sweeps included multiple levels of sine sweeps to characterize the nonlinear behavior of the rotation modes. The test, including all setup and teardown of instrumentation, was completed within three days. Fig. 17.1 Overall view of Roc outside of test facility [1]
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